The present invention relates to a semiconductor laser diode and manufacturing method therefor, and more particularly, to a ridge-type semiconductor laser diode having an improved ohmic contact surface and a manufacturing method therefor.
Recently, as the commercial availability increases for such laser devices for use in data processing systems and optical communication and other laser diode applications (e.g., laser printers and bar-code readers), there have been great improvements in the structure and manufacturing method of the laser diode as a light source. Specifically, many studies are underway for the ridge-type laser diode whose upper portion has a rectangular protrusion can be operated with a low threshold current. However, due to several reasons, the conventional ridge-type laser diode still has several problems in spite of the continuous study.
FIG. 1 is a section view showing the layers of a conventional ridge-type semiconductor laser diode. Referring to FIG. 1, the ridge-type laser diode is structured such that a first cladding layer 2, a first waveguide layer 3, an activation layer 4, a second waveguide layer 5, a second cladding layer 6, a cap layer 7, a passivation layer 8 and a current injection layer 9 are sequentially deposited on the ridge-type semiconductor substrate 1. Specifically, a rectangular ridge is formed in the center of the upper portion of second cladding layer 6. In addition, cap layer 7 is formed on the top surface of the ridge, passivation layer 8 covers areas excluding a portion of the top surface of the ridge, and current injection layer 9 resistively contacts cap layer 7 via an opening in passivation layer 8.
The operation of the laser diode can be explained as follows. First, when a voltage is applied to current injection layer 9, electrons are injected into the ridge through an ohmic contact surface 10 between the current injection layer 9 and cap layer 7. The electrons are coupled with the (electron) holes in activation layer 4, to thereby generate primary light which causes another recombination thereof. Thus, lights having the same frequencies are successively generated. The generated light is, while guided between first and second waveguide layers 3 and 5, resonates between mirrors-provided at the front and rear surfaces of the diode and thus oscillates as a laser.
A process for manufacturing such device is shown in FIG. 3A to FIG. 3F in sequence. Epitaxy layers such as first cladding layer 2, first waveguide layer 3, activation layer 4, second waveguide layer 5, second cladding layer 6 and cap layer 7 are sequentially grown on semiconductor substrate 1 (FIG. 3A). The growth process can be performed by various deposition methods such as metal organic chemical vapor deposition (MOCVD), plasma-enhanced chemical vapor deposition (PECVD) and molecular beam epitaxy (MBE). Then, on the second cladding layer 6, a silicon oxide (SiO.sub.2) mask layer is formed by employing a conventional patterning technique (FIG. 3B). Then, the second cladding layer is etched by employing a reactive ion etching (RIE) method so as to form a ridge (FIG. 3C). Then, the SiO.sub.2 mask is removed (FIG. 3D), and a passivation layer is deposited on the whole surface of the ridge structure (FIG. 3E). Here, the middle portion of the passive layer overlapped with the top end surface of ridge is etched by a patterning method to expose the epitaxy layers. Finally, an ohmic surface and a current injection layer are sequentially formed on the exposed surface of the ridge (FIG. 3F).
Such a conventional manufacturing method is apt to cause a misalignment of the passivation layer to the ridge. Therefore, a micro-aligning process is required when patterning for making the ohmic contact surface. Since, the ridge width is 2 to 3 .mu.m, and the actual width of the ohmic contact is approximately 1 to 2 .mu.m, an extremely precise aligning method is required, which often results in some deviation from the ridge and skewed etching.
FIG. 2 is a section view showing a laser diode where such misalignment has been generated in a manufacturing process. As shown in FIG. 2, when the opening of passivation layer 8 deviates from the top surface of the ridge, a current is injected from the side surface thereof instead of the top, which is fatal to the proper function on operation thereof. Further, there are other problems in the structure in that sheet resistance between layer 9 and ridge 3 is great due to the narrow ohmic contact therebetween via ohmic contact layer 7, and upon operation, the heat generated via the ohmic contact cannot be effectively transferred through ohmic contact layer 7 since the heat transfer of the laser diode performs mainly through the top rather than the side of the ridge. Therefore, it is important that the ohmic contact layer functions as a heat transfer medium. However, with the narrow ohmic contact surface area as above, the inefficient heat emission has a negative and often catastrophic influence on the life and operation stability of the device.
To solve the problems of misalignment, various self-aligning methods have been under study.
FIG. 4A to FIG. 4F illustrate a manufacturing process of a laser diode in which the ohmic contact surface is generated by a self-align method (Japanese laid-open Patent Publication No. 4-162689).
A first cladding layer 12, an activation layer 13, a second cladding layer 14, and a cap layer 15 are grown on a semiconductor substrate 11 having a ridge 11a with a planar top surface formed in the upper surface thereof (FIG. 4A). Then, a photoresist 3 having a high viscosity and a planar surface is deposited on the resultant structure (FIG. 4B), which is then etched by an oxygen plasma method to expose the top of ridge 15a (FIG. 4C). Then, the exposed upper portion of ridge 15a is removed by employing a solution of ammonia and hydrogen peroxide, which planarizes the upper surface of cap layer 15 (FIG. 4D). Then, a titanium film 2, i.e., a mask layer, is deposited on the upper surface of photoresist 3 and on the center of the exposed surface of cap layer 15 corresponding to the aperture in photoresist 3 (FIG. 4E). Next, the remainder of photoresist 3 (on both sides of ridge 15a) and titanium film 2 on photoresist 3 are removed together by employing a lift-off method. Thereafter, a proton injection (4) is performed over the entire substrate so as to form a high resistance region 5 on both sides of the remainder of titan film 2, to thereby complete the formation of the device (FIG. 4F).
The above-described method employs a self-alignment process which exposes the ohmic contact area via the opening portion of photoresist 3 without the need for micro-aligning. Thus, misalignment is not generated and the manufacturing process can be performed with precision. However, the process for forming a high resistance region is performed by complicated processes such as removing (planarizing) ridge 15a, depositing a titanium film and injecting protons. In addition, the heat emissive area of the device is limited to the two-dimensional top surface of the ridge. Therefore, the heat emission is not so efficient.
FIG. 5A to FIG. 5G illustrate a manufacturing method employing the self-align method disclosed in U.S. Pat. No. 5,208,183.
Referring to FIG. 5G, a gallium arsenide (GaAs) buffer layer 122, a first GaAlAs cladding layer 123, and a sandwich-structured (activation layer and waveguide layer) layer 124 are deposited on an n+GaAs substrate 121 whose bottom surface is provided with a metal (Au/Sn/Au) electrode 120. Then, a second GaAlAs cladding layer 125 whose upper surface is provided with a ridge, an SiO.sub.2 passivation layer 163 that covers the surface of second GaAlAs cladding layer 125 excluding the top surface of ridge 125a, a planarization layer 127, a metal (AuBe/Ti/Au) electrode 128, a p-GaAs layer 129 and a GaAs cap layer 130 are formed.
The manufacturing method of the above structure is as follows.
As shown in FIG. 5A, GaAs buffer layer 122, first GaAlAs cladding layer 123, the sandwich layer 124 with a activation layer and waveguide layer, and second GaAlAs cladding layer 125 are formed on a semiconductor substrate 121. Then, a mask 62 made of metal or a resist material is formed on the resultant structure.
As shown in FIG. 5B, the portion exposed on both sides of mask 62 is etched to a predetermined depth, and a ridge 125a is formed in second GaAlAs cladding layer 125.
As shown in FIG. 5C, a passivation layer 63 is deposited over the entire surface of the resultant structure.
As shown in FIG. 5D, a planarization layer 127 of polyimide is formed over the resultant structure.
As shown in FIG. 5E, the surface of planarization layer 127 is etched to a predetermined depth by the RIE method using an oxygen plasma, and passivation layer 63 formed in the top surface of ridge 125a is partially exposed.
As shown in FIG. 5F, the portion of passivation layer 63 which is exposed via the aperture of planarization layer 127 is etched, to thereby expose the surface of mask 62.
As shown in FIG. 5G, metal electrode layer 128 is deposited on mask 62, and the desired laser diode is obtained.
It is noticeable that the above-described manufacturing method can generate an ohmic contact surface by a self-alignment method without using any micro-aligning process, and is performed by a process which is more simple than that of the technique disclosed in the aforementioned Japanese laid-open publication. Such a laser diode obtained by the method, however, has several problems. For example, in the finally obtained structure, planarization layer 127 on both sides of the ridge serves as a thermal resistor and thus suppresses heat emission through both sides of the ridge upon operation. Thus, function degradation due to an inefficient heat transfer and emission and a heat concentration of the device is inevitable. Specifically, when a planarization layer is formed by polyimide, which is represented as an embodiment of the above method, degradation of the device caused by a thermal concentration becomes more serious due to the low thermal conductivity of the polyimide.
Another problem caused by including the planarization layer in the final structure is that a flexible material (polyimide) has to be employed for the planarization layer. Plastic deformation can be generated in such flexible materials during a cleaving process. In such a case, flexible materials may overlap the lasing region of the device, and thus block or scatter the light such that the desired function cannot be obtained.